The signals received by a radar which is intended chiefly to detect moving targets contain, up to the detection stage, echoes both from fixed targets and from moving targets.
In a fixed radar installation, echoes from fixed targets are characterized by Doppler frequencies of zero or virtually zero whereas echoes from moving targets are characterized by Doppler frequencies which are other than zero and which are related to the radial velocity of the targets. In the case of a moving installation, all the Doppler frequencies are shifted in proportion to the speed of the moving carrier.
Fixed echoes are usually eliminated by band-pass filtering of the video signals. The pass-band of the filter lies between the upper limit of the spectrum of the fixed echoes and the lower limit of the image of the spectrum of the fixed echoes, which is centered on the repetition frequency of the pulses.
Downstream of the filter, or bank of filters, which attenuates the fixed echoes but preserves the moving echoes, a threshold circuit of the bottom-clipping type eliminates any signal whose amplitude is below a predetermined value. This threshold is usually set at a level corresponding to the maximum amplitude of the residue of fixed echoes after filtering, so that any fixed echo has been completely eliminated behind the bottom-clipper. Only moving echoes whose amplitude is above the threshold are passed on.
The disadvantages of such an arrangement lie in its lack of flexibility and ineffectiveness in the presence of strong fixed echoes and weak moving echoes. In effect, throughout the range spread covered by the radar, there are ground echoes, generally few in number, of considerable amplitude, other echoes being of average or lower amplitude. The residue of a strong ground echo after filtering is of not inconsiderable amplitude. If this echo is not to appear on the display, it is necessary for the threshold of the bottom-clipping circuit to be positioned just above this amplitude. However, what this amounts to is eliminating from detection many moving echoes which are situated at various distances from the strong ground echo and which are surrounded by weaker ground echoes. One remedy for this disadvantage is deliberately to lower the threshold but in this case the number of false alarms increases.